As used herein, the term "Sense FET" means a semiconductor device comprised of two FET's (field effect transistors) in a single package, wherein one of the FET's (the smaller one) is designed to carry a predetermined percentage of the current carried by the other (larger) FET. If M1 refers to the larger FET and M2 refers to the smaller FET, then the current carried by M2 (IM.sub.2) can be described by the following equation: EQU Equation A:IM.sub.2 =K.sub.1 K.sub.2 IM.sub.1
where IM.sub.1 is the current carried by the larger FET M1, K.sub.1 is the size ratio of M1 and M2, and K2 is the ratio of the drain-to-source voltages (Vds) of M1 and M2.
In a typical application for a Sense FET, the larger FET M1 is used as a power device, and the smaller FET provides a source current that is measured and/or controlled in order to determine and/or control the level of current in the larger FET. FIGS. 1-3 show conventional approaches.
In FIG. 1, a sense resistor R.sub.s is coupled in series with the source current from M2, and the resultant voltage (Vsense) developed across R.sub.s is applied to one input of a comparator. A reference voltage is applied to the comparator's other input. Thus, when Vsense is equal to the reference voltage, the comparator develops an output signal which indicates that equality. A problem with this arrangement is that comparators typically have offsets of about 10 millivolts, thereby resulting in a relatively large error in the results of the comparison unless the offset is somehow compensated for.
In FIG. 2, a bipolar transistor Q1 is coupled across R.sub.s so as to limit the current in M1 and M2 when the voltage across R.sub.s is large enough to turn on Q1. A significant problem with this arrangement is that it consumes too much power. It can be seen that over two watts are dissipated when the value of R.sub.s is 0.2 ohms and 0.7 volts are required to turn on Q1.
In FIG. 3, an operational amplifier A1 senses the voltage across R.sub.s, but this configuration requires dual power supplies for the operational amplifier.